//1．修改程序清单17.2，让该程序既能正序也能逆序显示电影列表。
//一种方法是修改链表的定义，可以双向遍历链表。另一种方法是用递归。

/* films2.c -- using a linked list of structures */
#include <stdio.h>
#include <stdlib.h>      /* has the malloc prototype      */
#include <string.h>      /* has the strcpy prototype      */
#define TSIZE    45      /* size of array to hold title   */

struct film {
    char title[TSIZE];
    int rating;
    struct film * next;  /* points to next struct in list */
};
char * s_gets(char * st, int n);

void show_reverse(struct film * film);


int main(void)
{
    struct film * head = NULL;
    struct film * prev, * current;
    char input[TSIZE];

    /* Gather  and store information          */
    puts("Enter first movie title:");
    while (s_gets(input, TSIZE) != NULL && input[0] != '\0')
    {
        current = (struct film *) malloc(sizeof(struct film));
        if (head == NULL)       /* first structure       */
            head = current;
        else                    /* subsequent structures */
            prev->next = current;
        current->next = NULL;
        strcpy(current->title, input);
        puts("Enter your rating <0-10>:");
        scanf("%d", &current->rating);
        while(getchar() != '\n')
            continue;
        puts("Enter next movie title (empty line to stop):");
        prev = current;
    }

    /* Show list of movies                    */
    if (head == NULL)
        printf("No data entered. ");
    else
        printf ("Here is the movie list:\n");
    current = head;
    while (current != NULL)
    {
        printf("Movie: %s  Rating: %d\n",
               current->title, current->rating);
        current = current->next;
    }

    /* Program done, so free allocated memory */
    current = head;
    printf("反向显示：\n");
    show_reverse(head);
    while (current != NULL)
    {
        free(current);
        current = current->next;
    }
    printf("Bye!\n");

    return 0;
}

char * s_gets(char * st, int n)
{
    char * ret_val;
    char * find;

    ret_val = fgets(st, n, stdin);
    if (ret_val)
    {
        find = strchr(st, '\n');   // look for newline
        if (find)                  // if the address is not NULL,
            *find = '\0';          // place a null character there
        else
            while (getchar() != '\n')
                continue;          // dispose of rest of line
    }
    return ret_val;
}

void show_reverse(struct film * film){
    if (film==NULL) return;

    if (film->next != NULL) {
        show_reverse(film->next);
    }
    printf("Movie: %s  Rating: %d\n",
           film->title, film->rating);
}